The present invention relates to a power apparatus to supply electric power to equipment, systems and the like by the use of a rectifying system formed of a bridge rectifier circuit.
A variety of rectifying systems utilizing diodes has been known for long time. FIG. 7 shows a typical full-wave rectification circuit utilizing a bridge rectifier circuit. The full-wave rectification circuit of FIG. 7 has a bridge rectifier circuit formed of four of diodes 2 to 5. Reference numeral 11 indicates a load.
FIG. 7(a) is a diagram to show an electric current flow from alternating current power source 1 during negative half a cycle of the alternating current. The electric currents flow through diode 4, smoothing capacitor 7 and diode 3 in this order, thereby allowing positive voltage Vo to be tapped off.
More specifically, an alternating input from alternating current power source 1 is subjected to full-wave rectification and a positive direct current voltage is obtained.
However, a prior art power apparatus as described in above presents such problems as a low power factor and an increased content of power supply harmonics since input currents flow only during the period when the voltage of alternating current power source 1 is higher than the direct current output voltage.
As a general countermeasure to solve the foregoing problems, a method of connecting a reactor between alternating current power source 1 and bridge rectifier circuit 6 is adopted. Although this method allows the generation of harmonics to be suppressed, the power factor ends up with just reaching about 70% or so. As a result, in order to realize a power apparatus with a power capacity ranging from an intermediate capacity to a large capacity, there have risen such problems as causing the component elements used therein to become large in size with a resulting increase in the size of the power apparatus and taxing on the electric power source systems involved.
The present invention deals with the foregoing problems and aims at providing a power apparatus whereby both a high power factor and suppression of harmonics are allowed to be achieved.
A power apparatus of the present invention comprises:
an alternating current power source;
a bridge rectifier circuit to subject alternating currents from the alternating current power source to full-wave rectification; and
a smoothing capacitor connected across direct current output ends of the bridge rectifier circuit, further comprising:
a reactor connected between the alternating current power source and the alternating current input end of the bridge rectifier circuit;
a capacitor connected via a bi-directional switch between the alternating current input end and the direct current output end of the bridge rectifier circuit;
a zero-crossing detecting means to detect a zero voltage point of the alternating current power source;
a bi-directional switch drive signal generating means to generate a drive signal for the bi-directional switch based on an output of the zero-crossing detecting means; and
a bi-directional switch driving means to drive the bi-directional switch based on a signal of the bi-directional switch drive signal generating means.
According to the structure as described in above, turning on the bi-directional switch at a suitable phase and for a suitable conducting duration allows both the suppression of harmonics in input currents and the enhanced power factor to be realized and also allows a higher direct current output voltage than the peak voltage of the alternating current power source to be obtained. In addition, it is made possible for the value of the direct current output voltage to be controlled.
It is preferred that the power apparatus of the present invention is provided with a power source frequency detecting means to set specified time xcex94d from zero-crossing to a different fixed value based on power source frequencies, thereby allowing the control of the bi-directional switch to be facilitated.
It is preferred that the bi-directional switch drive signal generating means generates an on-signal at specified time xcex94d (0xe2x89xa6xcex94d) from a zero-crossing of the input voltage and generates an off-signal at specified time xcex94t (0xe2x89xa6xcex94t) after the on-time, thereby enabling the controlling of both the harmonics in input currents from the alternating current power source and the output voltage appearing across both ends of the smoothing capacitor.
It is further preferred that the variable range of specified time xcex94t after the on-time is limited to less than conducting duration xcex94 to of the bi-directional switch for generating a direct current output voltage needed at the time of a maximum load, thereby allowing the output voltage to be prevented from increasing to a higher value than necessary at the time of low loading.
It is preferred that the power apparatus of the present invention has a load detecting means and also has a storing means inside of the bi-directional switch drive signal generating means to store in advance a combination of specified time xcex94d from the zero-crossing and specified time xcex94t after the on-time, both being determined according to the magnitude of load, and further selectively retrieves from the storing means based on the output of the load detecting means a combination of specified time xcex94d from the zero-crossing and specified time xcex94t after the on-time according to the magnitude of load, thereby allowing the power apparatus to be operated constantly at the optimum point of performance against a variety of changes in loading condition.